Use of a gasket operating on the piston as a roll-back element of the latter is known from JP10292834, U.S. Pat. No. 6,044,936, U.S. Pat. No. 4,387,901, U.S. Pat. No. 3,421,604, U.S. Pat. No. 3,377,076 and EP0092896 A1.
This gasket is received in a seat of the surface of the piston seat or cylinder and presses on the piston so as to be deformed in the movement direction of the piston remaining with a surface at contact with the piston while the latter moves pressed by the pressurised brake fluid. Upon termination of the brake fluid pressure on the piston, the gasket deformed elastically and axially by the piston returns to the initial shape drawing the piston into the cylinder.
Should the pad impacted by the piston wear out during the braking action beyond the deformation limit of the gasket, the piston starts sliding with respect to the deformed gasket compensating the degree of wear of the pad. Upon termination of the braking action, the piston is returned by the gasket solely by the degree proportional to the deformation thereof.
Though satisfactory from various points of view, these known piston roll-back devices reveal inherent operation variability related to the fact that the gasket performs the double function of providing sealing between cylinder and piston and serving as an elastic element for the roll-back of the piston. The elastic roll-back is thus disturbed by the pressure of the brake fluid operating on the piston to an extent of the roll-back not being deemed constant over time. Furthermore the vicinity of the gasket to the source of heat which is generated by the friction between the disc and pad causes an unwanted temperature rise of the brake fluid with ensuing possible formation of vapour bubbles in the fluid. The high temperature can also cause a quick deterioration of the physical features of the material constituting the gasket thus leading—also in this case—to a high operation variability of the piston roll-back device.
Use of elastic means operating between two components constituting the piston to determine the roll-back thereof is known from WO07057929.
The device includes a piston comprising a piston body and a sleeve coaxial to said piston body, a scraper element received in a seat provided in the caliper body so as to press against the surface of the sleeve, elastic means being provided between said sleeve and said piston body. When the piston is pressed exiting from the seat thereof, it leads to a compression of the elastic means. Upon termination of the brake fluid pressure, the elastic means bring the piston body to an abutment position against the sleeve thus determining the roll-back of the piston by a predetermined degree.
This known piston roll-back device is more complex construction-wise due to the presence of several pieces constituting the piston and hence entailing a more difficult assembly.
Document WO00169102 shows a piston roll-back device similar to the previous WO07057929 wherein the seats of the gaskets operating on the cylinder are provided in the piston. Also in this case the complexity of the device entails difficult machining of the elements constituting the piston as well as a complex assembly thereof.